共查询到20条相似文献，搜索用时 31 毫秒
1.
Measurements of concentration fluctuation intensity, intermittency factor, and integral time scale were made in a water channel
for a plume dispersing in a well-developed, rough surface, neutrally stable, boundary layer, and in grid-generated turbulence
with no mean velocity shear. The water-channel simulations apply to full-scale atmospheric plumes with very short averaging
times, on the order of 1–4 min, because plume meandering was suppressed by the water-channel side walls. High spatial and
temporal resolution vertical and crosswind profiles of fluctuations in the plume were obtained using a linescan camera laser-induced
dye tracer fluorescence technique. A semi-empirical algebraic mean velocity shear history model was developed to predict these
concentration statistics. This shear history concentration fluctuation model requires only a minimal set of parameters to
be known: atmospheric stability, surface roughness, vertical velocity profile, and vertical and crosswind plume spreads. The
universal shear history parameter used was the mean velocity shear normalized by surface friction velocity, plume travel time,
and local mean wind speed. The reference height at which this non-dimensional shear history was calculated was important,
because both the source and the receptor positions influence the history of particles passing through the receptor position. 相似文献
2.
The Gaussian model of plume dispersion is commonly used for pollutant concentration estimates. However, its major parameters, dispersion coefficients, barely account for terrain configuration and surface roughness. Large-scale roughness elements (e.g. buildings in urban areas) can substantially modify the ground features together with the pollutant transport in the atmospheric boundary layer over urban roughness (also known as the urban boundary layer, UBL). This study is thus conceived to investigate how urban roughness affects the flow structure and vertical dispersion coefficient in the UBL. Large-eddy simulation (LES) is carried out to examine the plume dispersion from a ground-level pollutant (area) source over idealized street canyons for cross flows in neutral stratification. A range of building-height-to-street-width (aspect) ratios, covering the regimes of skimming flow, wake interference, and isolated roughness, is employed to control the surface roughness. Apart from the widely used aerodynamic resistance or roughness function, the friction factor is another suitable parameter that measures the drag imposed by urban roughness quantitatively. Previous results from laboratory experiments and mathematical modelling also support the aforementioned approach for both two- and three-dimensional roughness elements. Comparing the UBL plume behaviour, the LES results show that the pollutant dispersion strongly depends on the friction factor. Empirical studies reveal that the vertical dispersion coefficient increases with increasing friction factor in the skimming flow regime (lower resistance) but is more uniform in the regimes of wake interference and isolated roughness (higher resistance). Hence, it is proposed that the friction factor and flow regimes could be adopted concurrently for pollutant concentration estimate in the UBL over urban street canyons of different roughness. 相似文献
3.
4.
Eugene Yee P. R. Kosteniuk C. A. Biltoft J. F. Bowers 《Boundary-Layer Meteorology》1998,87(3):409-457
A series of tracer experiments studying the statistical properties of concentration fluctuations in clouds dispersing in the atmospheric surface layer is described and analyzed. Experiments were conducted at downwind fetches between about 200 and 1200 m, under a wide range of atmospheric conditions ranging from very unstable to moderately stable stratification. The present experiments have addressed basic requirements not met by past field experiments involving instantaneously released clouds; namely, the experiments provided repeat realizations of instantaneously released clouds measured with high-resolution concentration detectors, accompanied by the contemporaneous acquisition of high-quality meteorological and turbulence measurements.Extensive analyses are performed on the cloud concentration data in the framework of relative diffusion. Ensembles of cloud concentration realizations have been constructed. From these ensembles, crosswind and time profiles of the ensemble-mean concentration, concentration variance, ensemble-mean dosage, and dosage variance are obtained. The behaviour of the time profiles of the integral time scale of cloud concentration fluctuations is studied. The use of surface-layer similarity theory for the analysis of the downwind variation of a number of cloud quantities (e.g., cloud size and duration, cloud centre ensemble-mean concentration and dosage, cloud centre concentration and dosage variance, cloud centre integral time scale) is shown to be an effective basis for ordering these quantities. Furthermore, a number of approximate universal relationships describing the behavior of these cloud quantities has been derived. Finally, it is shown that the scaled crosswind and time profiles of ensemble-mean concentration and concentration variance as well as the scaled time profiles of the concentration fluctuation integral time scale exhibit self-similar forms that are independent of atmospheric stratification and downwind fetch. 相似文献
5.
A wind-tunnel study was conducted to investigate ventilation of scalars from urban-like geometries at neighbourhood scale
by exploring two different geometries a uniform height roughness and a non-uniform height roughness, both with an equal plan
and frontal density of λ
p
= λ
f
= 25%. In both configurations a sub-unit of the idealized urban surface was coated with a thin layer of naphthalene to represent
area sources. The naphthalene sublimation method was used to measure directly total area-averaged transport of scalars out
of the complex geometries. At the same time, naphthalene vapour concentrations controlled by the turbulent fluxes were detected
using a fast Flame Ionisation Detection (FID) technique. This paper describes the novel use of a naphthalene coated surface
as an area source in dispersion studies. Particular emphasis was also given to testing whether the concentration measurements
were independent of Reynolds number. For low wind speeds, transfer from the naphthalene surface is determined by a combination
of forced and natural convection. Compared with a propane point source release, a 25% higher free stream velocity was needed
for the naphthalene area source to yield Reynolds-number-independent concentration fields. Ventilation transfer coefficients
w
T
/U derived from the naphthalene sublimation method showed that, whilst there was enhanced vertical momentum exchange due to
obstacle height variability, advection was reduced and dispersion from the source area was not enhanced. Thus, the height
variability of a canopy is an important parameter when generalising urban dispersion. Fine resolution concentration measurements
in the canopy showed the effect of height variability on dispersion at street scale. Rapid vertical transport in the wake
of individual high-rise obstacles was found to generate elevated point-like sources. A Gaussian plume model was used to analyse
differences in the downstream plumes. Intensified lateral and vertical plume spread and plume dilution with height was found
for the non-uniform height roughness. 相似文献
6.
Kenneth R. Mylne 《Boundary-Layer Meteorology》1992,60(1-2):15-48
A set of tracer experiments studying concentration fluctuations in a pollutant plume dispersing near the surface in a stably stratified nocturnal boundary layer is described, and the results are compared with those obtained in near-neutral stability conditions by Mylne and Mason (1991). The results highlight the importance of slow meandering of the plume which is characteristic of stable conditions. This meandering makes it impossible to conduct experiments under near-stationary conditions, resulting in considerable statistical variability in the results, but is important in reducing time-averaged concentrations. Spectral characteristics of the plume and general fluctuation statistics are qualitatively similar to those in near-neutral stability, but there are significant quantitative differences. Fluctuation time scales are shown to be substantially longer under stable conditions. This difference cannot be fully explained by the reduced windspeed alone, indicating that the length scale of plume elements is also longer. Some of the differences observed in stable conditions, particularly the longer time scales, are shown to substantially increase the potential hazard due to fluctuations in practical applications. A conceptual model of plume dispersion is described, which explains the observed plume structure under different conditions by relating it to the turbulent velocity spectra. 相似文献
7.
8.
A wind-tunnel experiment was designed and carried out to study the effect of a surface roughness transition on subfilter-scale
(SFS) physics in a turbulent boundary layer. Specifically, subfilter-scale stresses are evaluated that require parameterizations
and are key to improving the accuracy of large-eddy simulations of the atmospheric boundary layer. The surface transition
considered in this study consists of a sharp change from a rough, wire-mesh covered surface to a smooth surface. The resulting
magnitude jump in aerodynamic roughnesses, M = ln(z
01/z
02), where z
01 and z
02 are the upwind and downwind aerodynamic surface roughnesses respectively, is similar to that of past experimental studies
in the atmospheric boundary layer. The two-dimensional velocity fields used in this study are measured using particle image
velocimetry and are acquired at several positions downwind of the roughness transition as well as over a homogeneous smooth
surface. Results show that the SFS stress, resolved strain rate and SFS transfer rate of resolved kinetic energy are dependent
on the position within the boundary layer relative to the surface roughness transition. A mismatch is found in the downwind
trend of the SFS stress and resolved strain rate with distance from the transition. This difference of behaviour may not be
captured by some eddy-viscosity type models that parameterize the SFS stress tensor as proportional to the resolved strain
rate tensor. These results can be used as a benchmark to test the ability of existing and new SFS models to capture the spatial
variability SFS physics associated with surface roughness heterogeneities. 相似文献
9.
We used wind-tunnel experiments to investigate velocity-field adjustment and scalar diffusion behaviour in and above urban canopies located downwind of various roughness elements. Staggered arrays of rectangular blocks of various heights H and plan area ratios λp were used to model the urban canopies. The velocity field in the roughness sublayer (height \({z \lesssim 2H}\)) reached equilibrium at distances proportional to \({\sqrt{L_{\rm c}H}}\) where L c is the canopy-drag length scale determined as a function of λp and the block side length L. A distance of about \({20\sqrt{L_{\rm c}H}}\) was required for adjustment at z = H/2 (in the canopy), and a distance of about \({10\sqrt{L_{\rm c}H}}\) was required at z = 2H (near the top of the roughness sublayer). Diffusion experiments from a ground emission source revealed that differences in upwind roughness conditions had negligible effects on the plume growth near the source (up to a few multiples of L from the source) if the source was located at a fetch F larger than about \({10\sqrt{L_{\rm c}H}}\) from the upwind edge of the canopy. However, at locations farther downwind (more than several multiples of L from the source), upwind conditions had considerable effects on the plume growth. For a representative urban canopy, it was shown that a much larger fetch than required for velocity-field adjustment in the roughness sublayer was necessary to eliminate the effects of upwind conditions on plume widths at 24L downwind from the source. 相似文献
10.
Pietro Salizzoni Lionel Soulhac Patrick Mejean Richard J. Perkins 《Boundary-Layer Meteorology》2008,127(1):97-110
Flow in the urban boundary layer is strongly influenced by the surface roughness, which is composed principally of isolated buildings or groups of buildings. Previous research has shown that the flow regime depends on the characteristic height of these obstacles (H), and the spacing between them (W). In reality, the urban boundary layer contains roughness elements with a wide range of length scales; in many practical situations these can be classified into large-scale roughness—buildings, or groups of buildings—and small-scale roughness, such as street furniture and elements on the façades and roofs. It is important to understand how the small-scale roughness might modify mass and momentum transfer in the urban boundary layer, but relatively little information is available concerning the potential interaction between large- and small-scale roughness elements in the different flow regimes. This problem has been studied using wind-tunnel experiments, by measuring vertical velocity profiles over a two-dimensional obstacle array, adding small-scale roughness elements to the top of larger parallel square bars. The experiments were performed for different cavity aspect ratios: the results show that the small-scale roughness increases the turbulence intensities and the momentum transfer when the large-scale obstacles are closely packed (H/W > 1) but it has very little effect for more widely-spaced obstacles (H/W < 1). 相似文献
11.
H. E. Eisma J. M. Tomas M. J. B. M. Pourquie G. E. Elsinga H. J. J. Jonker J. Westerweel 《Boundary-Layer Meteorology》2018,169(2):185-208
Simultaneous particle-image velocimetry and laser-induced fluorescence combined with large-eddy simulations are used to investigate the flow and pollutant dispersion behaviour in a rural-to-urban roughness transition. The urban roughness is characterized by an array of cubical obstacles in an aligned arrangement. A plane fence is added one obstacle height h upstream of the urban roughness elements, with three different fence heights considered. A smooth-wall turbulent boundary layer with a depth of 10h is used as the approaching flow, and a passive tracer is released from a uniform line source 1h upstream of the fence. A shear layer is formed at the top of the fence, which increases in strength for the higher fence cases, resulting in a deeper internal boundary layer (IBL). It is found that the mean flow for the rural-to-urban transition can be described by means of a mixing-length model provided that the transitional effects are accounted for. The mixing-length formulation for sparse urban canopies, as found in the literature, is extended to take into account the blockage effect in dense canopies. Additionally, the average mean concentration field is found to scale with the IBL depth and the bulk velocity in the IBL. 相似文献
12.
Drag due to regular arrays of roughness elements of varying geometry 总被引：13，自引：1，他引：13
Comparisons are made of experimental studies on the drag, at high Reynolds number, due to regular arrays of roughness elements
of various shapes immersed in a turbulent boundary layer. Using a variant of Millikan's dimensional analysis, the form of
the velocity profile is deduced in terms of the dimensions and concentration of the roughness elements. A drag formula results
which is shown to be in good agreement with data. Available measurements of the partition of drag between the elements and
the intervening surface indicates that equipartition occurs at quite low concentrations. The interaction between elements
is then small, so that the drag coefficient of a typical roughness element is nearly constant.
A re-examination of some of O'Loughlin's velocity-profile data, obtained below the tops of the roughness elements, suggests
the existence of a nearly constant-stress layer scaled to the shear stress of the intervening surface. Above the roughness
elements, the mean-velocity profile undergoes a transition to the form appropriate to the total shear stress exerted by the
roughened surface. A formula is given which describes the one-dimensional velocity profile over the entire range, excluding
the viscous sublayer on the intervening surface. The viscous sublayer appears to correspond quite closely to that on a smooth
plate.
This work was initiated while the authors were with the Division of Plant Industry, CSIRO.
At present on leave at the Natural Resource Ecology Laboratory, Colorado State University, Fort Collins, Colorado, U.S.A. 相似文献
13.
Markov Chain Simulations of Vertical Dispersion in the Neutral Surface Layer for Surface and Elevated Releases 总被引：1，自引：0，他引：1
John D. Reid 《Boundary-Layer Meteorology》1979,16(3):3-22
Vertical dispersion in the neutral surface layer is investigated using a Markov Chain simulation procedure. The conceptual
basis of the procedure is discussed and computation procedures outlined. Wind and turbulence parameterizations appropriate
to the neutral surface layer are considered with emphasis on the Lagrangian time scale. Computations for a surface release
are compared with field data. Good agreement is found for the variation of surface concentration and cloud height to distances
500 m downwind of the source. The functional form of the vertical concentration profile is examined and an exponential with
exponent ∼1.6 is found to give the best fit with simulations.
For elevated releases, it is demonstrated that an initial dip of the mass mean height from the simulation can be normalized
for various release heights using a non-dimensionalized downwind coordinate incorporating advective wind speed and wind shear.
The vertical distribution standard deviation (σz), as employed in Gaussian models, shows a fair degree of independence with source height but close examination reveals an
optimum source height for maximum σz at a given downwind distance,x. This source height increases with downwind distance. Also the simulations indicate that vertical wind shear is more important
than vertical variation of Lagrangian time scale close to the source, with a reverse effect farther downwind. 相似文献
14.
Drag partition measurements were made in the atmospheric inertial sublayer for six roughness configurations made up of solid
elements in staggered arrays of different roughness densities. The roughness was in the form of a patch within a large open
area and in the shape of an equilateral triangle with 60 m long sides. Measurements were obtained of the total shear stress
(τ) acting on the surfaces, the surface shear stress on the ground between the elements (τS) and the drag force on the elements for each roughness array. The measurements indicated that τS quickly reduced near the leading edge of the roughness compared with τ, and a τS minimum occurs at a normalized distance (x/h, where h is element height) of
(downwind of the roughness leading edge is negative), then recovers to a relatively stable value. The location of the minimum
appears to scale with element height and not roughness density. The force on the elements decreases exponentially with normalized
downwind distance and this rate of change scales with the roughness density, with the rate of change increasing as roughness
density increases. Average τS : τ values for the six roughness surfaces scale predictably as a function of roughness density and in accordance with a shear
stress partitioning model. The shear stress partitioning model performed very well in predicting the amount of surface shear
stress, given knowledge of the stated input parameters for these patches of roughness. As the shear stress partitioning relationship
within the roughness appears to come into equilibrium faster for smaller roughness element sizes it would also appear the
shear stress partitioning model can be applied with confidence for smaller patches of smaller roughness elements than those
used in this experiment. 相似文献
15.
北京海淀地区大气边界层的数值模拟研究 总被引：10，自引：1，他引：10
采用北京大学的三维复杂地形中尺度数值模式,利用Landsat-TM卫星影像图得到的地表状况计算陆气之间的能量平衡过程,模拟了北京海淀地区大气边界层的风、温场结构以及污染物浓度分布,进而模拟了由于汽车尾气的排放而转化成的气溶胶的浓度分布.模拟表明城郊之间存在热岛效应,边界层风场受到热岛的热力作用以及地形的动力作用影响.污染物及气溶胶浓度也存在城乡差别,最大浓度出现在城区的下风方向. 相似文献
16.
Velocity and Surface Shear Stress Distributions Behind a Rough-to-Smooth Surface Transition: A Simple New Model 总被引：1，自引：1，他引：0
A simple new model is proposed to predict the distribution of wind velocity and surface shear stress downwind of a rough-to-smooth
surface transition. The wind velocity is estimated as a weighted average between two limiting logarithmic profiles: the first
log law, which is recovered above the internal boundary-layer height, corresponds to the upwind velocity profile; the second
log law is adjusted to the downwind aerodynamic roughness and local surface shear stress, and it is recovered near the surface,
in the equilibrium sublayer. The proposed non-linear form of the weighting factor is equal to ln(z/z
01)/ln(δ
i
/z
01), where z, δ
i
and z
01 are the elevation of the prediction location, the internal boundary-layer height at that downwind distance, and the upwind
surface roughness, respectively. Unlike other simple analytical models, the new model does not rely on the assumption of a
constant or linear distribution for the turbulent shear stress within the internal boundary layer. The performance of the
new model is tested with wind-tunnel measurements and also with the field data of Bradley. Compared with other existing analytical
models, the proposed model shows improved predictions of both surface shear stress and velocity distributions at different
positions downwind of the transition. 相似文献
17.
Eugene Yee Ralph M. Gailis Alexander Hill Trevor Hilderman Darwin Kiel 《Boundary-Layer Meteorology》2006,121(3):389-432
We report on measurements of the near-field dispersion of contaminant plumes in a large array of building-like obstacles at three scales; namely, at full-scale in a field experiment, at 1:50 scale in a wind-tunnel simulation, and at 1:205 scale in a water-channel simulation. Plume concentration statistics extracted from the physical modelling in the wind-tunnel and water-channel simulations are compared to those obtained from a field experiment. The modification of the detailed structure of the plume as it interacts with the obstacles is investigated. To this purpose, measurements of the evolution of the mean concentration, concentration fluctuation intensity, concentration probability density function, and integral time scale of concentration fluctuations in the array plume obtained from the field experiment and the scaled wind-tunnel and water-channel experiments are reported and compared, as well as measurements of upwind and within-array velocity spectra. Generally, the wind-tunnel and water-channel results on the modification of the detailed plume structure by the obstacles were qualitatively similar to those observed in the field experiments. However, with the appropriate scaling, the water-channel simulations were able to reproduce quantitatively the results of the full-scale field experiments better than the wind-tunnel simulations. 相似文献
18.
地表空气动力学粗糙度是研究水平非均匀条件下陆面过程的基础,文中对黑河试验区各不同下垫面上地表空气动力学粗糙度进行了估算和分析,分别得到了地表空气动力学粗糙度ｚ０ｍ的有效值和局地值,发现各不同典型下垫面的局地ｚ０ｍ值明显不同,尤其在有作物存在时,绿洲的局地ｚ０ｍ值比沙漠、戈壁的值明显要大;而对同一测站,有效空气动力学粗糙度比局地空气动力学粗糙度值明显要大,运用应力分解理论对这种差异进行了解释,认为有效空气动力学粗糙度与复合表面的总应力有关,其代表了较大水平尺度内的表面和较高障碍物的综合作用,当涉及对陆表通量参数化问题时,必须首先确定水平尺度及相应的地表粗糙度参数 相似文献
19.
The effects of a pressure jump and a following internal gravity wave on turbulence and plume diffusion in the stable planetary
boundary layer are examined. The pressure jump was accompanied by a sudden increase in turbulence and plume dispersion. The
effects of wave perturbations on turbulence statistics are analysed by calculating fluxes and variances with and without the
wave signal for averaging times ranging from 1 to 30 min. The wave signals are obtained using a band-pass filter. It is shown
that second-order turbulence quantities calculated without first subtracting the wave perturbations from the time are greater
than those calculated when the wave signal is separated from the turbulence. Estimates of the vertical dispersion of an elevated
tracer plume in the stable boundary layer are made using an elastic backscatter lidar. Plume dispersion observed 25 m downwind
of the source increases rapidly with the arrival of the flow disturbances. Measured plume dispersion and plume centreline
height correlate with the standard deviation of the vertical velocity but not with the wave signal. 相似文献
20.
Jeffrey C. Weil Peter P. Sullivan Edward G. Patton Chin-Hoh Moeng 《Boundary-Layer Meteorology》2012,145(1):185-210
A Lagrangian particle dispersion model (LPDM) driven by velocity fields from large-eddy simulations (LESs) is used to determine the mean and variability of plume dispersion in a highly convective planetary boundary layer (PBL). The total velocity of a “particle” is divided into resolved and unresolved or random (subfilter scale, SFS) velocities with the resolved component obtained from the LES and the SFS velocity from a Lagrangian stochastic model. This LPDM-LES model is used to obtain an ensemble of dispersion realizations for calculating the mean, root-mean-square (r.m.s.) deviation, and fluctuating fields of dispersion quantities. An ensemble of 30 realizations is generated for each of three source heights: surface, near-surface, and elevated. We compare the LPDM calculations with convection tank experiments and field observations to assess the realism of the results. The overall conclusion is that the LPDM-LES model produces a realistic range of dispersion realizations and statistical variability (i.e., r.m.s. deviations) that match observations in this highly convective PBL, while also matching the ensemble-mean properties. This is true for the plume height or trajectory, vertical dispersion, and the surface values of the crosswind-integrated concentration (CWIC), and their dependence on downstream distance. One exception is the crosswind dispersion for an elevated source, which is underestimated by the model. Other analyses that highlight important LPDM results include: (1) the plume meander and CWIC fluctuation intensity at the surface, (2) the applicability of a similarity theory for plume height from a surface source to only the very strong updraft plumes—not the mean height, and (3) the appropriate variation with distance of the mean surface CWIC and the lower bound of the CWIC realizations for a surface source. 相似文献